3gb ram but only 1gb usable?

I have an older desktop, Acer Aspire running Windows 7 Ultimate 32 bit, it had 4gb ram installed (I did know that only 3gb could be used)

yesterday I took one of the ram sticks out ( had 4 x 1gb) and put in a different computer. Since then it has started crawling along, I have noticed that it says 3gb ram 1 gb usable. is this something that I need to go into the bios for? I ran the memory diagnostic tool this morning but did not get to see a report.